fix(vst): pin output bus to stereo (mode is decode-only) killing the hard-right pan; auto-default channel mode from the loaded capture (state v9)

tests/test_sampler_core.cpp

@@ -851,6 +851,35 @@ static void testMonoSamplePlaysDualMonoInStereo() {
     }
 }
 
+static void testDualMonoStereoSampleRendersCentered() {
+    // GA Bug 1 (pure-layer proof): a STEREO sample whose two channels are IDENTICAL (a
+    // dual-mono capture) must render EXACTLY equal L and R — bitwise, every frame — under
+    // BOTH pitch engines. Any asymmetry here (a silent L, a channel offset, divergent
+    // shifter state) would pan the output; hard-panned output from a dual-mono capture
+    // therefore cannot originate in the engine.
+    auto renderBoth = [](PitchEngine engine, int note) {
+        SampleData s = sineSample(600, 12.0, 60);
+        s.framesR = s.frames;  // dual-mono: identical channels
+        s.play.pitchEngine = engine;
+        Keymap km = Keymap::singleSampleChromatic(std::move(s));
+        VoiceEngine eng(1, km, /*preserveCap=*/0, /*preserveWindowFrames=*/128);
+        eng.noteOn(note, 127);
+        std::vector<AudioSample> left(256, 0.f), right(256, 0.f);
+        eng.render(left.data(), right.data(), 256);
+        bool sound = false, equal = true;
+        for (std::size_t i = 0; i < left.size(); ++i) {
+            if (left[i] != 0.0f) sound = true;
+            if (left[i] != right[i]) equal = false;  // EXACT: dual-mono must be centered
+        }
+        CHECK(sound);  // the render actually produced signal (a 0==0 pass would be vacuous)
+        CHECK(equal);
+    };
+    renderBoth(PitchEngine::Varispeed, 60);
+    renderBoth(PitchEngine::Varispeed, 67);  // off-root: repitch rides both channels equally
+    renderBoth(PitchEngine::Preserve, 60);   // both shifters run (no unity demotion in MIDI)
+    renderBoth(PitchEngine::Preserve, 67);   // off-root Preserve: per-channel shift, same state
+}
+
 static void testMonoRenderUnchangedByStereoData() {
     // Regression: the mono render path (renderFrame) reads channel 0 ONLY and is byte-identical
     // whether or not a second channel is present. A stereo sample rendered mono == its L channel.
@@ -1975,6 +2004,7 @@ int main() {
     testChannelCount();
     testStereoRenderKeepsChannelsDistinct();
     testMonoSamplePlaysDualMonoInStereo();
+    testDualMonoStereoSampleRendersCentered();
     testMonoRenderUnchangedByStereoData();
     testStereoRenderAdvancesLikeMonoRepitch();
     testStereoRenderSumsVoicesPerChannel();